The present invention is related to metal deposition processes as used for instance for the formation of conductive patterns connecting active or passive devices as well as integrated circuits. In particular, such conductive patterns can be formed at least partly by means of a plating deposition technique.
Electroless deposition has been used in the electronic packaging and the printed circuit board industry as a very efficient and cheap deposition technique for the production of thick films and for via-hole plating. To incorporate its use into the microelectronics industry, the reliability and quality of submicron lines and via""s generated by electroless deposition is a key issue.
The electroless deposition method involves the formation of a thin film of material from a plating bath without external electric current. The deposition is due to an electrochemical reaction between the metal ions, a reducing agent, a complexing agent, and pH adjusters on a catalytic surface. The deposition occurs by simultaneous oxidation-reduction reactions between two half reactions involving electron generation and electron reduction. The metal cations in the solution accept electrons at the deposition surface, get reduced, and are deposited as metal. A catalytic surface usually consists of either: 1) a surface which has been activated for instance with palladium nuclei, 2) a thin evaporated or sputtered seed of a noble metal like gold, platinum or palladium, or 3) an insulator (or non-catalytic semiconductor or metal) which is sensitised by coating it with a thin metal that can be less than a mono-layer. Once a thin layer of metal has been deposited onto the seed layer or sensitised surface, electroless deposition continues autocatalytically, since the metallic film is also a good catalyst for electroless growth.
For trench or via-hole plating, European Patent EP1020543 has proposed an original copper electroless plating bath fulfilling the stringent requirements for copper plating in sub-micro high aspect ratio features onto typical Cu diffusion barrier layer such as Ti, TiN, Ta, TaN, WNx, Ni, Ni-alloy and Co barrier-layers.
However, in order to obtain good quality electroless copper deposition on these barrier layers, first an activation step with deposition of palladium nuclei onto the barrier layer is required.
Moreover, the critical step for obtaining a catalytic surface to initiate copper deposition is still present.
Document WO 99/10916 claiming the priority of document U.S. Pat. No. 6,054,173 and related patents U.S. Pat. No. 6,054,172 and U.S. Pat. No. 6,126,989 disclose methods for eletroless deposition of copper on a titanium-containing surface of a substrate. One method for depositing copper on a substrate includes providing the substrate having a titanium-containing surface and forming a patterned catalyst material on the substrate, such that the titanium-containing surface is exposed in selected regions. Preferably, the catalyst material is selected from the group consisting of silicon, aluminium and chromium. The titanium-containing surface can be titanium or titanium nitride. Copper is then deposited on the exposed regions of the titanium-containing surface from an electroless solution preferably containing a source of copper and a reducing agent. The pH of said electroless solution is about 9 to 12.
One aim of the present invention is to initiate copper deposition from a plating solution directly on said barrier layers without requiring the above mentioned palladium activation step.
Another aim of the invention is to provide a new method for a conform and high quality copper deposition on barrier layers.
The invention discloses a new method for copper deposition on a substrate having a barrier layer characterized in that
a substrate is immersed in a copper plating bath, and that
an activator is also immersed in said copper bath and
direct contact is performed between said activator and said substrate for a predetermined period.
Preferably, the substrate is cleaned in a diluted HF solution, typically a 0.5 w %-10 w % HF solution, before it is immersed in the copper plating bath, in order to remove any native oxide.
A physical contact is necessary between the activator and the substrate for the copper deposition to occur.
The copper plating bath can be either a standard electroless plating bath or a plating bath without reducing agent.
The activator is preferably a contact metal which comprises at least an element selected from the group comprising Al, Zn and Pd.
Said activators may act according to two different mechanisms. When the activator is Al or Zn, it acts according to a first mechanism, wherein the activator has to be dissolved in the copper plating bath for copper deposition to occur on the substrate. When the activator is Pd, such a dissolution is not necessary. Pd acts according to a second mechanism, namely a self-limiting mechanism.
It has been established that Si and Cr can not work as activators.
The barrier layer is preferably selected from the group comprising a Ti layer, a TiN layer, a WNx, layer, Ta, TaN, Ni and Ni-alloy barrier-layers, a Co layer and Co-alloy barrier layers.
The copper plating bath preferably comprises at least:
a source of Cu (II) ions
a complexing agent to keep the Cu(II) ions in solution at high pH values
an additive to adjust the pH of said bath.
Preferably, said completing agent is diethyl-tartrate.
This copper plating bath can also comprise a reducing agent, preferably formaldehyde or hypophosphite.
The pH of the copper plating bath is comprised between 12.0 and 13.5. It has been shown that no copper deposition may occur if the pH of the copper plating bath is below 12.0.
The predetermined time period for direct contact between the two samples is preferably comprised between 10 sec. and 30 min.
The activator may have a determined shape. It can be in the form of a clamping ring or metal contacts placed on the wafer edge in contact with the barrier layer.
The method according to the present invention is preferably used for contact plated copper deposition on an opening (trench or via) in a substrate having a blanket barrier layer.